(1) Field of the Invention
This invention relates to a process for the hydroxylation of aromatic compounds and more particularly to a process for hydroxylating phenols and phenol ethers using hydrogen peroxide.
(2) Description of the Prior Art
Numerous process have been described for oxidizing phenols and phenol ethers with hydrogen peroxide in combination with metal salts, or with organic peracids (formed from hydrogen peroxide and a carboxylic acid). According to the circumstances, these process have allowed a hydroxyl radical to be introduced into the nucleus of the aromatic compound, or have brought about a more or less extensive oxidation of this nucleus, ranging from the production of quinones to the opening of the benzene ring with the formation of degradation products.
Thus, A. Chwala et al., J. Prakt. Chem. 152, 46 (1939) oxidized phenol with hydrogen peroxide in the presence of ferrous sulphate, in water acidified with sulphuric acid, to give a mixture of hydroquinone and pyrocatechol with a yield of 72% relative to the hydrogen peroxide used in the reaction. In spite of the good yields which it produces, this process has no industrial value on account of, firstly, the particularly prolonged contact times which it requires, which are the result of having to operate at the temperature of ice-cold water, and, secondly, the very great dilution of the reaction medium necessitated by this type of reaction. Moreover, it has been pointed out by Stein, J. Chem. Soc. 1951 3266 that reaction must be conducted under relatively mild conditions to avoid a violent reaction leading to benzoquinone.
G. G. Henderson et al., J. Chem. Soc. 91 1659.69 (1910), proposed oxidizing phenols with hydrogen peroxide in acetic acid which acted as solvent and provided peracetic acid. In the case of phenol, a reaction of several days at ambient temperature was necessary to obtain a mixture of hydroquinone, pyrocatechol and p-benzoquinone. Under analogous conditions, cresols lead to tarry products containing dihydroxymethylbenzenes.
The oxidation of various phenol ethers with organic peracids has also been carried out. Depending on the circumstances, these ethers were converted into quinones or even further oxidized with the opening of the aromatic ring, or, in some cases, were not even sensitive to the oxidizing agent, see S. L. Friess et al. J. Am. Chem. Soc. 74 1305 (1952); H. Fernholz, Chem. Ber. 87 578 (1954); H. Davidge et al., J. Chem. Soc. 1958 4569. Anisole and its homologues were not converted, or led to unidentified water-soluble products.
J. D. McClure et al. (J. Org. Chem. 27 627-8 (1962)) oxidized anisole and diphenyl ether in methylene chloride with trifluoroperacetic acid. The reaction did in fact allow phenolic hydroxy groups to be introduced into the aromatic nucleus, but the yields obtained were moderate; in the case of anisole, a mixture of isomeric methoxyphenols was obtained, with a yield of 34% relative to the phenol ether. However, the preparation of trifluoroperacetic acid is dangerous and can give rise to explosions.
U.S. Pat. No. 3,849,502 has proposed a process for the hydroxylation of phenols and their ethers by means of hydrogen peroxide, in the presence of traces of a strong acid and preferably with an agent which complexes the metal ions, such as orthophosphoric acid and polyphosphoric acid. Treatment of the phenol in accordance with this patented process gives a mixture of hydroquinone and predominantly of pyrocatechol.
Processes employing hydrogen peroxide associated with hydrofluoric acid have also been proposed for introducing a hydroxyl radical on an aromatic nucleus. U.S. Pat. No. 3,461,170 discloses the hydroxylation reaction carried out in the presence of substantially anhydrous hydrofluoric acid. The hydrofluoric acid process was then improved by adding a catalyst modifier such as alkanesulfonic acids or arylsulfonic acids as disclosed in U.S. Pat. No. 3,816,545 or trifluoroacetic acid as taught in published French Application No. 2,216,254 or by addition of a solvent chosen from the group consisting of the nitroalkanes, the aromatic nitro compounds, aniline and sulfolane as disclosed in U.S. Pat. No. 3,839,467. These processes make it possible to obtain better selectivity of the reaction, with respect to hydroquinone, when phenol is the aromatic compound hydroxylated. However, such processes require substantial amounts of hydrofluoric acid, which then becomes the reaction medium. In these large amounts the processes employing hydrofluoric acid are difficult to carry out since extreme care is required in handling this reactant. Furthermore, industrial development of such processes is hampered since hydrofluoric acid presents severe corrosion problems to equipment.
Thus, industry has no simple method available for introducing hydroxyl radicals into an aromatic nucleus with hydrogen peroxide. In particular, existing processes for hydroxylating phenols, particularly phenol itself, in aqueous media, require dilution conditions which remove all industrial character from these processes, and, when these conditions are not adhered to, do not allow the desired products to be obtained. Furthermore, the use of an excess acid, when hydroxylation is achieved with a peracid, complicates the recovery of the products formed.